Genetic analysis of many plant pathogen interactions has demonstrated that resistant plants often contain single loci that specify resistance against pathogens containing complementary avirulence genes. This genetic pattern is the basis of the "gene-for-gene" hypothesis which states that plant disease resistance is controlled by a single dominant resistance locus that encodes a product(s) that specifically interacts (directly or indirectly) with the product of a corresponding avirulence gene. Elucidation of the function of the gene products encoded by these loci has been impeded by the absence of cloned genes. In the bacterial blight disease of rice, both the host (Oryza sativa) and pathogen (Xanthomonas campestris pv. oryzae, Xco) are amenable to molecular genetics, making this an attractive system for studies of disease resistance. Only one locus, Xa21, confers resistance to all races of Xco. The gene-for-gene hypothesis predicts that Xco strains contain a single locus, avrXa21, that confers avirulence to the pathogen. The specific objectives of this proposal are: 1) To construct a high resolution genetic map of the plant resistance locus Xa21; 2) To identify and analyze mutants of the Xa21 line of rice that are susceptible to infection by Xco; 3) To construct a physical map of the Xa21 genomic region and isolate overlapping yeast artificial chromosome clones containing Xa21; 4) To clone and characterize the Xa21 gene(s); 5) To identify chemical and transposon induced mutants of Xco that can cause disease on Xa21 plants; and 5) T clone the avrXa21 avirulence gene by complementation of the mutant phenotype. Molecular characterization of the host and pathogen genes that specify disease resistance will help answer questions regarding the complexity of the resistance locus Xa21. Cloning of Xa21 and avrXa21 would represent the first time that both a disease resistance gene and the corresponding avirulence gene have been isolated from any crop. Analysis of the gene products encoded by these loci will contribute to a basic understanding of signal transduction pathways, protein-protein interactions, and control of gene expression. Finally, having the gene cloned raises the possibility of expanding the resistant germplasm of other crop species through gene transfer experiments.